For most people, boredom is a passing, nearly trivial feeling that lifts as soon as your number is called, a task is completed or a lecture ends. But boredom has a darker side: Easily bored people are at higher risk for depression, anxiety, drug addiction, alcoholism, compulsive gambling, eating disorders, hostility, anger, poor social skills, bad grades and low work performance.

Despite boredom's ubiquity and pathological associations, psychologists have yet to pin down what, exactly, it is. Several different scales all claim to measure boredom—the most widely used is the Boredom Proneness Scale—but a recent analysis suggests that they are measuring slightly different phenomena. Explanations for ennui are even more plentiful, ranging from Freud's theories of repressed emotions to individual differences in personality traits, the need for excitement, and attention skills.

Part of the boredom puzzle may be individual differences in how much excitement and novelty we require. Men, for example, are generally more bored than women. They also exhibit more risk-taking behaviors, report enjoying more dangerous entertainment and are more likely to say that their environments are dull. "People who are more likely to become bored do not see their environments as very rich or lively," says Stephen Vodanovich at the University of West Florida in Pensacola, who has been working on boredom for almost 20 years.

Clues to the underlying causes of boredom have come from patients who suffer traumatic brain injuries (TBI). According to James Danckert, a neuroscientist at the University of Waterloo in Ontario, people with TBI often begin to indulge in riskier activities after their accidents. These activities might include taking drugs or jumping out of planes&mash;pursuits they pick up in an attempt to deal with their new and chronic boredom.

Danckert theorizes that the massive flux of endorphins or pain medication necessary for recovery from a brain injury may have literally raised these patients' threshold for psychological pleasure and reward. "Now instead of a coffee doing it for you, you need a triple espresso," Danckert explains. "Anything that used to give you pleasure now has to be ramped up in order to succeed." Like chronically bored but healthy people, they need far bigger hits to find fun.

Highly bored individuals also tend to lack the ability to entertain themselves. As a result, they may turn to activities like doing drugs, says McWelling Todman at the New School for Social Research in New York City. "Drug use takes place during downtime when the person would have otherwise been entertaining themselves." This may be especially true during adolescence, a time "when they are putting together the skills needed to deal with boredom in adulthood." Boredom therefore becomes a lifelong cue for sensation-seeking behavior. If drug addicts can learn to deal with their doldrums, however, they may be less likely to relapse. In one as-yet unpublished study of 156 addicts ranging in age from 24 to 68 at a methadone clinic, the subjects' reported levels of boredom were the only reliable factor that predicted whether they would stay on course, Todman notes.

Our culture's obsession with external sources of entertainment—TV, movies, the Internet, video games—may also play a role in increasing boredom. "I think there is something about our modern experience of sensory overload where there is not the chance and ability to figure out what your interests, what your passions are," says John Eastwood, a clinical psychologist at York University in Toronto.

It is possible that the roots of boredom lie in a fundamental breakdown in our understanding of what it is we want to do. Bored people tend to score low on measures of self-awareness. They find it difficult to accurately monitor their own moods and feelings and hence understand what they truly want. These findings fit into the psychodynamic model of boredom, whereby people repress their true wants and desires and therefore cannot locate satisfying activity. The repression part is still debatable, but Eastwood has found that students who scored high on scales of alexithymia—difficulty in describing or identifying feelings, distinguishing between bodily sensations and feelings, and an inhibited inner emotional and fantasy life—also tended to be bored.

At a more functional level, the ability to focus or engage also plays a significant role in boredom. People with attention deficit hyperactive disorder (ADHD) are more likely to be bored, as are those who score low on measures of sustained attention. So, too, are individuals who have brain injuries or are prone to flips of attention (such as driving on autopilot or putting the milk in the cupboard). In fact, direct manipulation of attention can lead to boredom. In a classic experiment from 1989, James Laird at Clark University in Worcester, Mass., found that low-level distraction—by way of a quiet television in the next room—caused participants to find a reading task "boring."

These results support the idea that we label tasks as boring when they require a great deal of focused effort to hold our attention. In the 1970s, psychologist Mihaly Csikszentmihalyi, who is now at the Claremont Graduate University in Claremont, Calif., coined the term "flow psychology"—the idea that great absorption, focus and enjoyment of work results from a balance between our skills and the challenge of the tasks we face.

Both tasks that are too dull, such as factory work, or too complicated, such as doing taxes, feel tedious. Of course we all differ in our ability to focus, see the beauty and complexity of our surroundings, or ascribe meaning to our actions. We also differ in our interest or knowledge of an area. "You might love something that I would consider incredibly mundane," Vodanovich says. "I have some good friends who love classical music and I cannot stand it."

Boredom, like so many quirks of the brain, remains a mystery. One step toward unraveling it would be to develop better tools to measure boredom. There might even be different types, ranging from the existential, always present ennui to the transient, toe-tapping kind. As a result, different explanations may apply to different situations. "It is about bringing together all these different areas and trying to make some kind of synthesis," Danckert says. "Is it an emotion that we can't understand? I think we can understand it much better than we currently do. But like all things in the brain—there will be some parts of it that continue to elude us."